| Summary: | 博士 === 國立成功大學 === 電機工程學系碩博士班 === 96 === Optical code-division multiple-access (OCDMA) offers high statistical multiplexing gain in a bursty traffic environment and is thought to be a more suitable solution in local-area network. Early incoherent optical CDMA systems used pseudo-orthogonal sequences to encode signals in the time domain, but the codes were long and multiple access interference (MAI) limited the number of simultaneous users. Thus, spectral-amplitude-coding (SAC) optical CDMA systems were proposed to eliminate the influence of MAI.
In this dissertation, an optical spectral compensation scheme is proposed to flatten the chips spectra over arrayed-waveguide grating (AWG) router-based code-division multiplexing (CDM) network. Spectral coding chips are attained by slicing the spectrum from a broadband light source (BLS) and then coded with AWG router to form stream of spectral chips. The chips spectra sliced from the contemporary BLS are usually not flat and will induce coding chips with unequal power levels. The resulting unequal chips power will cause MAI at the receiver. Chips spectra compensation is therefore examined with the backward-pumped fiber Raman amplifiers (FRAs). The thesis realizes optical equalizer by adopting genetic algorithm (GA) and linear matrix to flatten the power spectra of SAC-OCDMA chips by identifying the optimal pump wavelengths and pump power of backward-pumped FRAs. Signal-to-interference ratio (SIR) is evaluated with such spectral compensation and performance improvement is found to be in the range of ~20 dB.
For increasing the OCDMA network capacity, this thesis proposes a novel SAC-OCDMA scheme with four optical intensity levels by polarization–division -multiplexing (PDM) to increase the maximum permissible number of simultaneous subscribers using an optical broadband source of finite bandwidth. In the proposed system, each encoder/decoder pair is shared by two subscribers and the requirement for complex polarization compensator schemes at the receiver is removed. The phase-induced intensity noise (PIIN) induced at high optical intensities significantly degrades the performance of a conventional SAC-OCDMA scheme. Therefore, this thesis uses two methods to suppress the PIIN effect, namely (1) two users share the same encoder and transmit signals on mutually orthogonal polarizations such that some PIIN terms are canceled out; and (2) the receiver structure is equipped with multiple balanced detectors (MBDs) in order to reduce the PIIN power impinging on each photodiode (PD). The numerical evaluation results demonstrate that under PIIN limited conditions, the proposed system achieves a higher performance than that of the conventional SAC-OCDMA scheme.
Two different methods are proposed to realize a double-spread CDMA scheme for radio-over-fiber (RoF) transmissions. One method is the network coder/decoders (codecs) are implemented using AWG routers coded with maximal-length sequence (M-sequence) codes. The effects of PIIN and MAI on the system performance are evaluated numerically for different values of the optical modulation index (OMI) during the nonlinear electro-optical modulator (EOM) response. The influence of the degree of polarization (DOP) in the system is also discussed. By employing the scrambler in front of the balanced photo-detector, the system performance can be enhanced.
The second method is optical network codecs are implemented using optical switches (OSWs) coded with bipolar pseudo-noise (PN) codes. The MAI caused by nonlinear optical source on the system performance are evaluated numerically for different values of the OMI. At low OMI optical device noise is dominant, but at high OMI nonlinear effect becomes significant. The high-performance, low-cost characteristics of the double-spread CDMA render the scheme an ideal solution for radio-CDMA wireless system cascaded with optical CDMA network.
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